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| Home > Publications database > Investigation of electrocatalysts by in-situ energy dispersive X-ray absorption and scattering |
| Book/Report | FZJ-2019-01614 |
1996
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/21712
Report No.: Juel-3284
Abstract: Cyclic voltammetry, X-ray Absorption Near Edge Spectroscopy (XANES ) and, for the first time, Anomalous Small Angle X-ray Scattering (ASAXS) have been used to study electrooxidation processes in carbon supported platinum electrocatalysts. The study was performed with synchrotron radiation at the JUSIFA ASAXS beamline, HASYLAB / DESY. The samples were prepared from commercial carbon supported catalyst powder with platinum contents of 5, 10, 60 and 80 wt.%. The powder was mixed with 30 wt.% Teflon and deposited on conductive 0.15 mm thick carbon paper as porous working electrodes for in-situ measurements in a specially designed, flat X-ray transparent electrochemical cell. The electrolyte was 1M H$_{2}$SO$_{4}$ at room temperature. XANES spectra and ASAXS data near the X-ray Pt-L$_{3}$ absorption edges were recorded at controlled potentials 0.25 for the reduced and 1.1 V vs. Ag / AgCl for the oxidized catalyst states. In the absorption experiments, the X-ray absorption spectra ofthe sampie as well as of metallic Pt sheets and PtO$_{2}$ as reference sampies were recorded. By comparison with the referencesampies, the heights of the white lines in the XANES absorption spectra were used to determine the extent of the oxidation. A strong dependence of the extent of oxidation on the Pt-particle sizes was observed. Under the same condition, a larger extent of oxidation occurs in the smaller particles. The actual loading of the catalysts, typical 1 to 2 mg/cm$^{2}$, was determined from the edge jump in the absorption spectrum. Independently, consistent values for the Pt loading were obtained from the small angle scattering data. In the small angle scattering experiments, anomalous scattering was applied to separate the scattering of the platinum catalyst particles from the background scattering of the porous carbon support. The separated scattering cross sections were fitted with lognormal distributions for theparticle sizes. The mean sizes ofthe platinum particles for 5, 10, 60 and 80 wt. % Pt /C under study were found to be 1.70 ± 0.02 nm, 1.72 ± 0.02 nm, 7.28 ± 0.56 nm and 36.4 ± 2.4 nm for the reduced state and 2.08 ± 0.06 nm, 2.12 ± 0.06 nm, 8.36 ± 0.56 nm and 35.2 ± 2.0 nm for the oxidized state. For the smaller size, the shift of the size distribution to larger sizes for the oxidized particles was found to result from the formation of oxide shells at the particle surfaces. The limit of the method is reached for the largest particle size, where the small expected shift remains within the error bars. [...]
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